1. Field of the Invention
The invention relates in general to waveguide circulators for the non-reciprocal transmission of microwave energy; and more particularly to a novel system for reducing the size, mass, and insertion loss of the transition from a first circulator to either a second circulator or to a terminating load.
2. Description of the Related Art
Multi-junction waveguide ferrite circulator assemblies have a wide variety of uses in commercial and military, space and terrestrial, and low and high power applications. A waveguide circulator assembly may be implemented in a variety of applications, including but not limited to LNA redundancy switches, T/R modules, isolators for high power sources, and switch matrices. Ferrite circulators are desirable for these applications due to their high reliability, as there are no moving parts required. This is a significant advantage over mechanical switching devices. In most of the applications for multi-junction waveguide switching and non-switching circulators, small size, low mass, and low insertion loss are significant qualities, for example, in satellites where redundancy switches are desired directly behind an antenna array.
A commonly used type of waveguide circulator has three waveguide arms arranged at 120 and meeting in a common junction. This common junction is loaded with a non-reciprocal material such as ferrite. When a magnetizing field is created in this ferrite element, there will be a gyromagnetic effect that can be used as a switching action of the microwave signal from one waveguide arm to another. By reversing the direction of the magnetizing field, the direction of switching between the waveguide arms is reversed. Thus, a switching circulator is functionally equivalent to a fixed-bias circulator but has a selectable direction of circulation. RF energy can be routed with low insertion loss from one waveguide arm to either of the two outputs arms. If one of the waveguide arms is terminated in a matched load, then the circulator acts as an isolator, with high loss in one direction of propagation and low loss in the other direction. Reversing the direction of the magnetizing field will reverse the direction of high and low isolation.
For applications where additional isolation is required between waveguide ports or where additional input/output ports are required, multiple waveguide circulators and isolators are used. The most basic building blocks for multi-junction waveguide circulator networks are single circulator junctions and single load elements, both optimized for an impedance match to an air-filled waveguide interface. For the purposes of this description, the terms “air-filled,” “empty,” “vacuum-filled,” or “unloaded” may be used interchangeably to describe a waveguide structure. The circulators and loads can be connected in various configurations as required for the desired isolation and input/output port configuration. For circulator and isolator junctions, the direction of circulation may either be fixed or switchable.
Conventional waveguide networks comprised of multiple ferrite elements typically have impedance-matching transitions between the ferrite elements. For example, conventional waveguide circulators may transition from one ferrite element to a dielectric-filled waveguide such as a quarter-wave dielectric transformer structure, to an air-filled waveguide, and then back to another dielectric-filled waveguide section and the next ferrite element. The dielectric transformers are typically used to match the lower impedance of the ferrite element to that of the air-filled waveguide. There are several disadvantages to utilizing transformers in such a manner. When dielectric transformers are used, RF losses can be introduced in various ways, such as the following: losses in the dielectric material itself, increased losses in the waveguide surfaces due to the high concentration of RF currents on the metal waveguide surfaces disposed directly above and below the dielectric transformer element, and losses in the adhesives typically used to bond the transformers to the conductive housing.
The use of dielectric transformers also takes up additional space in the waveguide structure. This increases the minimum separation distance that can be obtained in multi-junction assemblies when the input/output ports of multiple circulators are intercoupled to provide a more complex microwave switching or isolation arrangement. This can result in a multi-junction waveguide structure that is undesirably large and heavy.
Just as the standard transitional sections from one ferrite element to another occupy a significant amount of space in traditional multi-junction waveguide circulator networks, so do the transitions from a ferrite element to an absorptive load. These load elements are required to absorb the power that passes through the ferrite element in one direction when the circulator is used as an isolator. Although decreased loss is not an issue for the absorptive load design, decreased size and mass are still desirable attributes of the design.
U.S. Pat. No. 4,697,158 (the '158 patent) discloses one method for decreasing the spacing and loss between the ferrite elements by replacing the standard dielectric transformers with a reduced height waveguide transition. This method removes the transformers, but the reduced height transition is sensitive to dimensional variations, which results in a design that is expensive and difficult to manufacture and assemble. Additionally, the reduced height transition design requires the presence of a significant gap between the ferrite elements, which increases the size of the component.
In view of the problems with the conventional waveguide circulator structures disclosed above, there is a need for a multi-junction waveguide circulator structure with improvements in the critical areas of size, mass, cost, and insertion loss.